The invention relates to a method for reducing the noise in a signal from a motion detector and further relates to a circuit arrangement for carrying out the method.
For the reduction of noise in video signals circuits are known which are based on the principle of time recursive filtering. When an output signal delayed by one picture period is fed back to the input of a recursive filter, the noise component is averaged over a plurality of picture periods and is thus reduced. The degree of noise reduction then depends on the magnitude of the fed-back picture signal in proportion to the input signal.
In order to preclude blur as a result of the recursive filter in the case of moving picture scenes, DE-A-31 21 597 discloses a system for the reduction of noise in a television signal, which system comprises a motion detector to control the magnitude of the fed-back video signal in dependence upon the motion occurring in the picture. In the known system, a motion signal is generated by forming the difference between the non-delayed and the delayed video signal. However, similarly to the input signal, this motion signal initially contains noise. Since this noise should not be applied to the recursive filter, it is suppressed by means of a threshold circuit and a low-pass filter. However, if this threshold circuit is not set to the correct threshold value, the recursive filter will fail to operate correctly. The known system provides a correction of the threshold value by noise measurement and rectification, but it is also responsive to movements, which leads to errors.